Assays of chemical reactions, utilizing techniques such as, for example, scintillation counting, luminometry, fluorimetry and kinetics measurements, are carried out on a large scale in both industry and academia. As a result, it is desirable to have an apparatus that allows such assays to be performed in a convenient and inexpensive fashion.
Because they are relatively easy to handle and low in cost, microplates are commonly used for performing assays of chemical reactions. Microplates typically consist of a plurality of individual wells. An aliquot of a sample to be assayed is placed in each well. The wells are usually arranged in relatively close proximity in a matrix pattern. Common sizes for microplates include matrices having dimensions of 4.times.6 (24 wells) or 8.times.12 (96 wells), although larger microplates are also used that may include matrices of hundreds or even thousands of wells. Microplates are preferably arranged to allow each sample to be assayed independently of the other samples.
In certain assay methods frequently performed in wells (e.g, chemiluminescence), a chemical reaction is measured by the emission of visible light. Similarly, in assay methods such as fluorimetry, visible light is emitted when a sample is exposed to a specific wavelength of radiation to commence the chemical reaction. When performing assay methods involving the measurement of visible light from the samples, optical cross-talk may occur between microplate wells. Optical cross-talk is an interference in signal measurement that results from the transmission of photons of visible light between wells. Thus, the result measured from a particular well may be skewed by light emitted from a different sample. Therefore, optical cross-talk should be minimized because it negatively impacts the ability to assay samples individually.
Despite the harmful effects of optical cross-talk on the data produced by these assay methods, there exist relatively few microplates designed to significantly reduce this effect. It has been recognized that to reduce optical cross-talk between the wells of a microplate, certain regions of the wells such as the sidewalls should Be formed from material that does not allow the transmission of visible light therethrough. However, since light measurements are typically taken from the bottom surface of the wells, the bottom surface must generally be formed from a material that is transparent to visible light. Thus, optical cross-talk can occur through the transparent material that forms the bottom surfaces of the wells.
One attempt to reduce optical cross-talk in microplate wells is disclosed in U.S. Pat. No. 5,298,753 to Sonne et al, which discloses a microplate having wells that include a light permeable bottom plate and light impermeable sidewalls. To reduce the problem of optical cross-talk between adjacent wells, the lower surface of the bottom plate is provided with an adhesive tape having a grid of black lines positioned beneath the areas where the bottom. sheet contacts the light impermeable sidewalls. The black lines are said to reduce optical cross-talk by blocking some light rays that would otherwise pass between adjacent wells. However, because the adhesive tape is attached to the lower surface of the bottom plate, it necessarily cannot prevent the exchange of light between adjacent wells through the layer of material that forms the bottom layer. Therefore, Sonne concedes that the disclosed technique is only 90% effective in reducing optical cross-talk. Furthermore, given the laboratory environment in which this microplate is likely to be used, the adhesive may not be sufficiently durable to withstand repeated use.
Another example of a microplate designed to reduce optical cross-talk is disclosed in U.S. Pat. No. 5,319,436 to Manns et al. This microplate also includes an opaque upper plate that forms sidewalls for each well, and a transparent lower plate. The upper and lower plates have relatively complex designs so that they may be mated together and sonically bonded to form the microplate. The opaque material includes a bead surrounding each well, and extending partially into the transparent lower plate. It is said that the opaque beads should extend through 25-75% of the thickness of the lower plate, and that they provide a barrier to light transmission between adjacent wells. However, because the beads do not extend entirely through the transparent lower plate, light transmission between adjacent wells through the bottom plate is not completely eliminated. Furthermore, the relatively intricate designs of the upper and lower plates that must be mated together and sonically bonded result in a comparatively expensive manufacturing process.
It is an object of the present invention to provide an improved microplate for use in assaying samples and an improved method for manufacturing microplates.